The present invention pertains to ejector pins for ejecting a molded article from a mold cavity, and more particularly, pertains to such ejector pins for use without liquid lubricants and to method of making and using the same.
In many plastic molding applications, it has become increasingly necessary to mold plastic parts in a production process that is free of oil, grease, or other liquid lubrication. This type of lubrication migrates to the molding surface and ultimately the molded part. Typical examples of where this is an unacceptable occurrence would be plastic parts for medical applications, parts for the food packaging industry, and clear acrylic parts for lenses or displays.
Injection molders, therefore, have had three options for contending with this requirement. One way is to run a standard pin dry in the core insert. Because of side loads experienced between the pin and insert, wear and galling could then occur. Another way would be to lubricate the pin with grease and then have an operator at the molding press visually look at each part and perform a secondary cleaning operation. This obviously is labor intensive and, therefore, not cost effective, and not a positive means of assuring that the part will be free from grease.
The third way that this has been addressed is to secondarily treat a standard ejector pin with a dry lubricating treatment or plating after the pin has been machined and ground to a finished length. This has had to be done secondarily on a xe2x80x9cper moldxe2x80x9d basis, because most lubricous treatments are not sufficiently heat resistant and abrasion resistant to withstand the process the mold maker uses to cut, deburr, and finish grind the length of the ejector pin. That is, the typical cutting operation results in the cut end becoming red hot and this could cause a deterioration in some coatings; and the cut end is formed with an outward projecting burr. This burr is ground off, and there is a finish grinding of the pin end which could peel off or flake off coatings applied to the pin. When a treatment is applied secondarily to a finished length ejector pin on a xe2x80x9cper moldxe2x80x9d basis, then that processing is both inconvenient and expensive as opposed to treating the parts during the production manufacturing stage.
There is a need for nominal sized ejector pins having a friction reducing treatment which remains intact following the cutting and deburring of the ejector pin, so that builders of plastic injection molds and die cast dies can quickly and economically purchase a pre-treated pin xe2x80x9coff the shelfxe2x80x9d.
Mold builders buy standard size ejector pins in lengths of 6xe2x80x3, 10xe2x80x3 or the like, and then cut the pins close to the desired dimensions that are usually in hundreds of an inch, e.g., 8.756 inch. After cutting, they xe2x80x9cdeburrxe2x80x9d, or file away the burrs, created by cutting the end of the ejector pin. Mold builders will often, when it is required that the mold is to run without lubricant, cut their purchased nominally sized ejector pins, deburr the cut pin ends, finish grinding the pin ends to provide the exact length for the pin and send the cut and deburred ejector pins out to a plater to coat them with a coating to assist in their sliding without liquid lubricant. Customized coating of each differently cut size of ejector pins is costly and time consuming. One attempt to overcome these problems was made by a company that coats standard pins with manganese phosphate coating which lowers friction initially but fails to last long. This process produces a crystalline bond that is traditionally used with liquid lubrication during the initial break-in period of moving metal components. It is not a permanent dry lubricant, but generally used as a means of retaining liquid lubrication. Another company coated their pins with titanium nitride. After cutting and deburring a titanium nitride coated ejector pin, an exposed, substrate white layer of material was present at the cut end of the ejector pin. With this substrate exposed at the cut end, the substrate may peel or flake because of the attack of resin gasses and heat which may reach approximately 600xc2x0 F. The peeling or flaking will create a cavity into which the plastic will flash. There is a need for a dry coating for nominal length ejector pins which can be effective after the mold maker cuts and deburrs the coated ejector pins much in the manner that the mold maker does with the conventional non-coated ejector pins.
One problem with conventional, batch-coated pins is their inability to withstand corrosive gases released during curing of certain plastic resins. For example, polyvinylchloride (xe2x80x9cPVCxe2x80x9d) gives off strongly corrosive gases which make the use of conventional batch-coated chromium pins unacceptable for such applications. Moreover, chromium coatings tend to be more expensive than is desirable.
The dry coating of the ejector pin should have the ability to be applied in a substantially uniform thickness and within a close tolerance, have good low-friction characteristics, have good adhesion to the ejector pin, have good abrasion resistance to remain intact during deburring, have good resistance to withstand resin gases and other chemicals, and have acceptable heat resistance for temperatures encountered in injection molds.
It was found that an extremely thin coating of a hard, low friction metal, such as chrome and/or nickel, i.e., 0.00005 inch to 0.0001 inch, applied to the ejector pins provides a dry lubrication which facilitates low friction sliding of the ejector pins with respect to the mold. The thin treatment has also been found to remain intact at the ends of the pins after cutting and deburring of the ends of the ejector pins, while providing the desired low friction sliding of the pins through the mold. Nominal length pins of 6xe2x80x3, 10xe2x80x3 or 14xe2x80x3 may be purchased by mold builders having a thin coating of the treatment, allowing the molders to cut the pins to any desired length particular to their specific mold configurations, and deburr the cut end of the pins, with the treatment remaining intact without chipping or flaking. This eliminates the need for molders to send their ejector pins out for coating following cutting to size, and thus reduces the costs associated with customized ejector pins and speeds a mold to production. Because the coating is so thin, it does not appreciably increase the diameter of the ejector pin. Thus, the same ejector pin, whether coated or uncoated, may be used in the same mold. If the coating were a substantially thick coating, then diameter of the steel pin would have to be reduced; and a second inventory of steel ejector pins of a diameter smaller than the diameter of normal uncoated, steel pins, to be used with liquid lubricants, would have to be made and kept. For this application, coating of greater than 0.0002xe2x80x3 may be considered unacceptable because if the coating were to peel or chip off, it would be a substantial enough thickness to result in flash of the molded material. Manifestly, the coated pins could be lubricated and used in lubricated molds, if so desired; and they would hold up better than the uncoated pins if proper lubrication were not maintained.